The lowest of the lowest. There is a rat and then there is.... a proper rat. A proper rat is a full blown slut/ganga usually walking the streets at night looking for it. A "Proper Rat" is down for whatever (sexually) and not in a good way but in a "hey i just gave you a venereal disease" way.

"Bro did you see that proper rat at that station?"
"Yeah blud, she said she would give me a gobby for a ciggie."
"Fuck that bro, you'd be scratching your knob forever after that shit."
"Word"